As has been said, a tuner is your biggest bang for the buck. If you want to take full advantage of what a tuner can give you need to help the air flow in and out. Plan on about $1500.00 for a basic tuner, intake and exhaust mods.
At minimum you need a good flowing intake system, the best being a HAMMER. Many CAI's just make more noise. If you can get a stock Daytona/SRT air box for cheap you will do almost as good as some of the expensive A/M systems.
Once you get the air in you need to get it out. Just taking of the rear resonators with Blast'n Bob resonator eliminators will help with the flow and sound good too. You can do the Brylcream mod (suitcase removal) and get the same effect. You best route is to get a good Cat Back system with a tuned X pipe or center muffler. The JBA system is the best for the buck.
For you basic intake, exhaust tuner mod you should be able to take about a half a second off your stock 1/4 mile time and run over 100 MPH. With headers you should shave another 3-4 tenths off and be in the 13.5's at around 105 MPH
With a single shot nitrous I'm running 12.8's at 110 MPH. I hope to see 12.5's at around 115 MPH with my 2 stage system ( Friday 11/2 should be the day) I have a K&N drop in filter in a SRT air box, FlashPaq Tuner, 180 T/stat, Cooling Fan Mod, Kooks L/T headers, Hi Flow Cats and JBA cat back exhaust.